This invention relates generally to computer input devices, and more particularly to digital image capture devices used to provide ranging and tracking information for a computer system.
The range of an object, i.e. the distance to the object from an observation site, can be determined by the analysis of two or more spatially separated images (often referred to as xe2x80x9cbinocular imagesxe2x80x9d when there are two images) that are taken from the observation site. In range computation from simultaneously acquired binocular digital images, the area of processing is limited to the visible region of overlap between the two images. To maintain a reasonable region of overlap usually necessitates redirecting the optical axes of the cameras (i.e. changing their vergence) which introduces other problems including a usual necessity to resample the imagery. As is well known to those skilled in the art, xe2x80x9cvergencexe2x80x9d means the angle between the optical axes of the lenses.
The processing of binocular or multi-view imagery for range computation is easiest when the optical axes are parallel and the imaging surfaces are coplanarxe2x80x94in what is termed parallel epipolar geometry. Because verging the optical axes to optimize the region of image overlap eliminates image-surface co-planarity, the complexities of calculating range increases significantly with non-parallel viewing. This is further compounded when viewing objects at a variety of azimuths and distances where adjustments in the view direction as well as verging would be necessary to retain sufficient image overlap.
For computational purposes, the frame of reference for scene description is usually tied to image location, so changing the image location through vergence adjustments necessitates reconfiguring the frame of reference. Again, adjusting a system""s frame of reference increases the computational and conceptual complexity of its analysis.
A similar situation arises for typical monocular (i.e. single image) computer analysis of tracking and scanning in some space before the camera. With subjects able to operate over a broad region before the camera, continued observation generally involves use of either wide-angle optics or a panning/tilting mechanism to properly direct the camera""s view direction. These control mechanisms are relatively complex, must move fairly large pieces of equipment (cameras and their lenses), and alter the underlying geometric frame of reference for scene analysis by rotating the frame of reference with the cameras. In addition, the use of wide angle optics works against high resolution analysis, as only larger scene detail is visible.
One approach to solve these acquisition problems in image-based range and tracking computation would be to employ greatly oversized imagers (e.g. imagers having about 3K by 3K or 9xc3x97106 elements), and select corresponding standard-sized windows within these for processing. However, such an approach would be prohibitively expensive. For example, a 1K by 1K imager sells for well over a thousand dollars. Higher resolution imagers are available at considerably greater price.
A prior art solution to the apparent dichotomy between simple processing (with parallel epipolar geometry) and broad depth and tracking coverage exists in adaptation of perspective-correcting lens systems as used in xe2x80x9cview-cameraxe2x80x9d or xe2x80x9ctechnical-cameraxe2x80x9d designs. In such designs, an oversized lens is used to image the scene, and lateral repositioning of the lens or imaging platform can be used to redirect the camera without rotating the imaging surface. For single camera use this enables maintaining lines parallel in the world parallel on the image plane; in ranging camera use it enables parallel epipolar geometry.
For example, in U.S. Pat. Nos. 5,063,441 and 5,142,357 of Lipton et al., devices for use in 3D videography are disclosed. More particularly, Lipton et al. teach devices for capturing binocular images for use in stereo videography (stereo movies), with reduced viewing eyestrain, by using dual and triple camera systems. Briefly stated, Lipton et al. teaches an imager controller for epipolar stereo capture in videography, including stereo lenses mounted fixedly together in a single housing. Stereographics Inc., of San Raphael, Calif., produces a product embodying elements of the Lipton et al. patents.
In the matter of two-dimensional imager control, U.S. Pat. No. 5,049,988, of Sefton et al. teaches a system that provides the display of a video capture window for surveillance applications. Phillips, in U.S. Pat. No. 4,740,839, teaches a TV surveillance system operated by sub-sampling a conventional camera, with a result that resembles the Lipton et al. approach of image capture.
As will be appreciated, image capturing of the prior art uses planar sensors due to the high cost, lack of availability, and complexities involved with the use and manufacture of curved or xe2x80x9csphericalxe2x80x9d sensors. However, spherical sensors have a number of advantages with respect to field of view, view direction, and use in stereo image capture that designers of prior art digital imaging cameras have apparently failed to consider.
The present invention includes a multi-image camera system for automated stereo ranging applications that provides high-resolution broad field coverage over a wide range of subject distances while enabling retention of the epipolar constraints necessary for efficient stereo matching. At the same time, the multi-image camera of the present invention supports monocular image applications such as object tracking.
In one embodiment of the present invention, an imaging device is preferably placed on a three-degree-of-freedom linear motion positioning mechanism whose position can be controlled by a computer. Broad field coverage is preferably attained through use of a wide-angle large-coverage lens (e.g. a lens designed for a 35 mm camera). High resolution is attained through placing the imaging device under the large-coverage lens, so the imaging device""s immediate field of view is considerably narrower than that provided by the lens.
In contrast with traditional view-camera usage, the present invention teaches moving the imaging surface instead of the lens. This allows the apparatus to retain co-planarity of the images and, so long as the displacements can be determined to sub-pixel accuracy, maintains a stable frame of reference for the analysis, all while providing the required view re-directions. Moving the lens alters the projective relationships among observations, whereas moving the imager does not.
Image focus may be attained through the traditional rotating-travel focus adjustment although, for quantitative computational tasks, this is often unsatisfactory as the center of projection may vary with lens rotation. For this situation, our preferred embodiment of the moving imager provides a back-focus capability, as will be described below.
Computer control of the platform enables positioning accuracy in the micron range. Stepper motors, for example, operating with full step positioning, can position a platform along an axis to within a few microns of the desired position. Differential stepper control enables increasing the accuracy of this placement (the number of locations where this precision is attainable) by one to two orders of magnitude. Fine specification of absolute imager location can also be attained through use of interferometers or sonar-based ranging systems, as will be described below in more detail.
The planar imaging embodiment of the present invention enjoys a number of advantages over related devices of the prior art. In the present invention, an oversized lens is used to provide a wide field of view of the scene. Movement of the imaging surface under the lens provides high-resolution view selection, which is the equivalent of view redirection of pan and tilt motions without the need for a complex mechanism for accomplishing such motions. With accurate positioning knowledge, this form of imaging provides a stable frame of reference for scene computation. Accurate positioning information may be attained using the positioning and control systems described below.
Furthermore, the apparatus of the present invention provides an economical solution to the aforementioned problems of the prior art. For example, the present invention is operable with relatively economical high-precision imagers that use displacement for direction selection in tracking, scanning, and range computation. More particularly, certain embodiments of the present invention place an imaging device on a three-degree-of-freedom linear motion platform whose position can be controlled by a computer to efficiently and economically provide the desired direction selection. The third degree of freedom provides back-plane focus control. Back-plane focusing has an advantage for traditional lens designs where focus from element rotational displacements causes image centering variations.
In a spherical camera embodiment of the present invention, image capture is effected through spherical focal-plane imaging using, for example, a spherical-faced fiber-optic faceplate. One lens suitable for this embodiment, the Baker Ball lens, is described in a wartime report referenced below. In this spherical-imaging embodiment of the present invention, a mechanism is provided that shares many of the design considerations of the above-described linear moving-imager camera. Preferably, this embodiment uses a high resolution imaging device (1K by 1K elements with 12 bits of resolution at each pixel), mounted behind a fiber-optic faceplate that transfers the focal-plane light pattern to the sensor elements of the imager. Notable in this embodiment of the moving imager is that the lens system has no particular optical axis or axis along which any imaging surfaces are preferentially oriented.
In addition, the spherical camera embodiment of the present invention has a number of advantages over prior art planar imaging cameras. For example, the spherical camera embodiment we describe has 1) excellent optical resolution attainable since the focal surface does not need to be made planar (the lens is diffraction limited); 2) greater simplicity due to the advantages of rotational over linear displacements; 3) a greatly increased undistorted field of view; 4) exponentially less lens illumination fall-off with eccentricity (cosine of radial displacement rather than the fourth power of the cosine); 5) greater effective pan and tilt velocities; and 6) opportunity to study (via simulation) aspects of human psychophysical behavior in a mechanism of similar geometry.
Certain embodiments of the present invention, both of the linear and spherical lens, provide lenses in multiple housings to increase the flexibility, reliability, and range finding quality of the system. A monocular version of the present invention requires only a single lens, and multi-lens versions of the present invention can use two or more lenses, for multi-image ranging analysis.
These and other advantages of the present invention will become apparent upon reading the following detailed descriptions and studying the various figures of the drawings.